[tex]d = \sqrt{ {L}^{2} + {l}^{2} + {h}^{2} } [/tex]
[tex]25 = \sqrt{ {9}^{2} + {12}^{2} + {h}^{2} } [/tex]
[tex]25 = \sqrt{81 + 144 + {h}^{2} } [/tex]
[tex]25 = \sqrt{225 + {h}^{2} } \: |( \: )^{2} [/tex]
[tex] {25}^{2} = 225 + {h}^{2} [/tex]
[tex]625 = 225 + {h}^{2} [/tex]
[tex] {h}^{2} = 625 - 225[/tex]
[tex] {h}^{2} = 400[/tex]
[tex]h = \sqrt{400} [/tex]
[tex]h = 20 \: cm[/tex]